The San Juan River placers stretch over a distance of 30 kilometres from the junction of Clapp and Floodwood creeks to the east down to Mosquito Creek in the west. Most creeks on the south side of the San Juan River, flowing north off of San Juan Ridge, are reported to carry placer gold.
The area is underlain by metamorphic (slaty schists) and bimodal volcanic rocks of the Jurassic to Cretaceous Leech River Complex. The San Juan River Fault separates these from intrusive rocks of the Paleozoic to Jurassic Coast Crystalline Complex and volcanic rocks of the Lower Jurassic Bonanza Group to the north.
Placer gold occurs almost exclusively in the gravels of the streams that drain the area that is underlain by the slaty schists of the Jurassic to Cretaceous Leech River Complex (Formation). Fairly coarse gold may be found in the gravels of virtually all these streams. The gold in recent gravel deposits is likely derived from the numerous quartz veins that occur in the slaty schists. These veins are seldom more than small stringers and lenses a few centimetres wide and approximately 1 metre in length. The only metallic minerals in the veins are a little pyrite or chalcopyrite and free gold. The veins are generally too small and too barren to be profitably mined.
Bulletin 21, from 1946, states that placer miners have worked the following rivers and streams of Vancouver Island: China and Loss creeks, and Leech, Gordon, Jordan, Sooke, Sombrio, San Juan, Bedwell, Nanaimo, Gold, and Zeballos rivers.
It appears that most of the gold was derived from bars or in crevices in the bedrock of the river bed, or from benches along the side of the river.
The placer deposits were discovered in the late 1800’s in association with placer gold exploration on the island, following the discovery of the Leech River Placers (MINFILE 092B 078) in the 1860’s. Minor work by local prospectors has continued through to present.
In 1990, panned concentrate sampling, by Beau Pre Explorations, yielded visible grains of gold from Mosquito Creek and two of its tributaries (Assessment Report 20050).